There has been a recent increased emphasis on the use of unmanned aerial vehicles for performing various activities in both civilian and military situations where the use of manned flight vehicles is not appropriate and/or feasible. Such missions include surveillance, reconnaissance, target acquisition and/or designation, data acquisition, communications relay, decoy, jamming, harassment, ordnance delivery, or supply flights. This increased emphasis on the role of UAV's in today's (and tomorrow's) society has led to many advancements in both airframe design and propulsion systems.
There are generally three types of UAV configurations under current development, a fixed-wing type configuration (a fuselage with wings and horizontally mounted engines for translational flight), helicopter type configuration (a fuselage with a rotor mounted above which provides lift and thrust) and ducted type configuration (a fuselage with a ducted rotor system which provides translational flight, as well as vertical take-off and landing capabilities). A wingtype UAV provides several benefits over a helicopter or ducted type UAV. First, and foremost, is the ability of a winged UAV to travel at considerably greater speeds and for longer distances than a conventional helicopter or ducted type UAV. Also, a winged UAV can typically carry a larger mission payload and/or fuel supply than a helicopter or ducted type UAV. As such, fixed-wing UAVs are generally better suited than helicopter or ducted type UAVs for certain mission profiles involving endurance, distance, higher speed and load capability.
Winged UAVs, however, have deficiencies that limit their utility. For example, since winged UAVs require forward motion to maintain lift and therefore are not capable of hovering over a fixed spatial point. As a result, winged UAVs are not very good at delivering ordinance or laser designating targets. Also, winged UAVs cannot take-off and land vertically. Instead, winged UAVs require elaborate launch and retrieval equipment.
Helicopter UAVs can hover over a fixed spatial point and takeoff and land vertically but have limitations when operating in confined areas due to the exposed rotors rotating above the fuselage. Also, helicopter UAVs tend to have a high center-of-gravity (CG) and therefore have limited ability when landing on sloped surfaces or pitching ship decks. A high CG aircraft tends to roll over when landing on steep slopes.
The ability of ducted rotor-type UAVs to take-off and land vertically, combined with their ability to hover for extended periods of time over a point and operate in confined areas off steep slopes, make a ducted type UAVs ideally suited for real time tactical reconnaissance, target acquisition, surveillance, and ordnance delivery missions for front line tactical units.
Ducted-type UAVs, such as the CYPHER.RTM. unmanned aerial vehicle developed by Sikorsky Aircraft Corporation and generally disclosed in U.S. Pat. No. 5,152,478, includes a toroidal fuselage shrouding co-axial, counter-rotating rotors. The rotors are designed to provide the thrust necessary for both vertical and translational flights. As shown in FIG. 1A, aircraft vertical motion of the UAV is provided by maintaining the vehicle fuselage substantially horizontal so that the thrust (downwash) of the rotors provides the necessary lift for the aircraft. When fore-aft or lateral movement of the aircraft is desired, the aircraft fuselage must be "nosed-down" as shown in FIG. 1B in order to generate a horizontal thrust component.
As discussed above, ducted-type UAVs have a relatively slow speed as compared to winged UAVs. One reason fot this is that most ducted-type UAV's do not have a separate translational propulsive system. As such, the rotor system must provide both vertical and translational thrust, thus, requiring the full potential of the rotor system to be split.
Another problem associated with a toroidal UAV relates to drag. Referring back to FIG. 1A, if the aircraft were to fly in the forward direction (i. e., to the left in the figure) while oriented horizontally, the airflow passing over the nose N of the aircraft would impact the inner rear wall D.sub.W of the duct. This generates considerable drag on the aircraft. To reduce the drag on the aircraft, it is oriented as shown in FIG. 1B. This orientation of the aircraft causes the airflow to pass through the rotor system, reducing airflow contact with the duct wall D.sub.W.
Another problem associated with conventional ducted-type UAVs is the tendency of the aircraft to experience a nose-up pitching moment. That is, the airflow over the airframe and through the rotor system produces a moment about the aircraft's center of gravity which causes the nose of the aircraft to pitch upward. There have been several attempts made to counter-act this nose-up pitching moment. U.S. Pat. No. 5,152,478 discloses a UAV rotor system wherein cyclic pitch is used to counter-act the nose-up pitching moment during forward translational flight. Although this solution does eliminate the nose-up pitching moment, it also requires a considerable amount of power and does not eliminate the drag on the duct wall.
Another possible option to counter-act the nose-up pitching moment is to optimize the toroidal fuselage airfoil profile. The utilization of an optimized toroidal fuselage airfoil profile to counteract the nose-up pitching tendency of UAVs is disclosed in U.S. Pat. No. 5,150,857. This solution requires that the outer aerodynamic surface of the toroidal fuselage be optimized to provide an asymmetrical toroidal fuselage pressure distribution that produces high lift forces during forward translational flight modes. The high lift forces reduce the required lift provided by the rotor assembly, thereby reducing the undesirable nose-up pitching moment. A reduction in required power is effected by the decreased requirement for rotor lift and the reduced need for superimposed cyclic pitch (moment trim).
While the incorporation of a toroidal fuselage having an optimized outer aerodynamic surface represents a viable option to help counteract the fuselage-induced nose-up pitching moments, this option incurs a manufacturing penalty and may have an adverse effect on higher speed flight characteristics.
A further solution to reducing the nose-up pitching moment in a ducted-type UAV is disclosed in U.S. Pat. No. 5,419,513 wherein ancillary wing structures are incorporated onto the aircraft to counter-act the nose-up pitching moment. More particularly, the ancillary wing structures have an aerodynamic configuration that generates lifting forces to supplement the lifting forces generated by the rotor assembly and the toroidal fuselage.
Although the incorporation of ancillary wings on the aircraft does help counter the nose-up pitching moment caused by translational flight, the aircraft translational thrust is still limited by the amount of horizontal thrust component that can be generated by the rotor system.
A need, therefore, exists for an improved rotor-type UAV which provides increased propulsive capabilities and reduced drag during forward translational flight.